Windows CI: clean up legacy clcache cache after #4472 #4477

#4472 moved the clcache cache to C:\clcache_dir and left the old %USERPROFILE%\clcache behind.
Since then the Windows workers have been running out of disk: x64-2 hit 100%, and every reliability report from September 19 to September 24 lists disk-space failures, with none before.
The workers were already low on disk before #4472, so the old directory may not be the only cause.

#4485 deletes the old directory on every compile.

If the workers are still tight after that, the next step is the active cache's 20 GB limit (clcache -M 20000000000 in compile.cmd), which ignores free disk space, ccache, and the workspaces on C:.
I can't check disk usage myself, because my account has no Jenkins Overall/Read permission.

Related: #4442, #3897.

Assisted by OpenAI Codex and Claude Code; I reviewed and verified everything myself.